This invention relates to radar apparatus comprising a transmitter for a radio signal the frequency of which repeatedly sweeps from a first frequency to a second frequency at a first substantially constant rate and also sweeps from a third frequency to a fourth frequency at a substantially constant rate which has the same magnitude as, but is of opposite sign to, said first substantially constant rate, a mixer for mixing any return signal received back at the apparatus after reflection by a target or targets with a sample of the transmitted signal to produce a beat signal the frequency spectrum of which has a component corresponding to the or each target, a frequency analyser for analysing said frequency spectrum to determine the frequency of the or each said component, and means for determining the difference between the frequencies of the component corresponding to a given radially moving target arising from a sweep in the frequency of the transmitted signal from the first frequency to the second frequency and the component corresponding to the same target arising from a sweep in the frequency of the transmitted signal from the third frequency to the fourth frequency
Apparatus of this general kind is known from, for example, GB-B-2172461. If one considers merely the frequency sweeps of the transmitted signal from the first frequency to the second frequency, and reflection by a single stationary target, then the result of the mixing process is a beat frequency signal (constant if sweep end effects are neglected) the frequency f of which is proportional to the target range r. In fact r=cf/2.alpha. where c is the velocity of light and .alpha. is the rate of change of frequency of the transmitted signal during each sweep. Analysis of the frequency spectrum of the beat frequency signal will therefore yield information about the range r of the target. If, however, the target is moving in such a manner that it has a component of motion towards or away from the radar apparatus the frequency of the reflected signal will be respectively greater than or less than it would be if the target were stationary, due to the Doppler effect. If the frequency sweeps of the transmitted signal are, for example, in an upward direction, this will result in the frequency of the beat frequency signal being respectively less than or greater than it would be if the target were stationary. This means that it is possible for a stationary target at a given range, a target at a greater range which is moving towards the radar, and a target at a shorter range which is moving away from the radar to all give rise to beat frequency signals having the same frequency; the relationship between this frequency and the actual range of the relevant target is uncertain due to the unknown Doppler frequency shift f.sub.d. In fact ##EQU1## if the frequency sweeps of the transmitted signal are in an upward direction, and EQU r=c(f.sub.2 -f.sub.d)/2.alpha. (1B)
if the frequency sweeps of the transmuted signal are in a downward direction, where f.sub.1 and f.sub.2 are the beat frequencies obtained in the respective cases, f.sub.r is the portion of these beat frequencies due to the range r, and c and .alpha. have the same meanings as defined hereinbefore. This uncertainty can be resolved, as is known, by sweeping the frequency of the transmitted energy in both directions. Provided that the centre frequencies f.sub.o of the upwards and downwards sweeps are the same the mean Doppler frequency shifts f.sub.d due to a given moving target will be the same in both cases. However, as will be seen from equations 1A and 1B, their effects on the frequencies of the beat frequency signals obtained will be oppositely directed in the two cases, so that, provided that the upward and downward sweep rates are the same, the actual range of the target can be determined by taking the average of the beat frequencies f.sub.1 and f.sub.2 respectively obtained in the two cases, and/or the actual Doppler shift, and hence the actual radial velocity v of the target, can be determined by taking (half) the difference between the beat frequencies obtained in the two cases. Expressed mathematically: EQU 2r=c(f.sub.1 +f.sub.d)/2.alpha.+c(f.sub.2 -f.sub.d)/2.alpha.. EQU or r=c(f.sub.1 +f.sub.2)/4.alpha. (2)
and ##EQU2## (For the purpose of (3) the direction of the velocity v has been assumed to be towards the radar apparatus).
The frequency sweeps of the transmitted signal are normally obtained by deriving this signal from a voltage-controlled oscillator (VCO) or the like, the control input of this oscillator being fed with voltage ramps from the output of a sawtooth or triangular wave generator, for example a periodically reset integrator circuit fed with a constant predetermined input voltage or a simple integrator circuit fed with first and second predetermined input voltages alternately. The value of the or each predetermined input voltage together with the time constant of the integrator circuit then determines the slope of each voltage ramp and hence, via the frequency-versus-control-voltage characteristic of the oscillator, the rate of change .alpha. of the oscillator output frequency during each frequency sweep. Whilst it is not too difficult to set and maintain the slopes of the voltage ramps generated by such circuits at the required value over considerable periods of time, stability of the frequency-versus-control-voltage characteristic of a VCO over comparable periods of time is much harder to achieve, with the result that the rate of change .alpha. of the oscillator output signal may vary in an unpredictable manner. As this factor .alpha. appears in the equation relating target range to beat frequency or frequencies obtained, this relationship may also vary in an unpredictable manner so that inaccuracies occur in the target ranges measured by the radar apparatus. It is an object of the invention to mitigate this problem.